Method for preparing monocrystalline silicon and solar cell and photovoltaic module with monocrystalline silicon

ABSTRACT

Provided is a method for preparing a gallium- and nitrogen-doped monocrystalline silicon using a Czochralski process, including: introducing a doping gas at least including a first amount of nitrogen into a molten mixture in a single crystal furnace; withdrawing a seed from the molten mixture while introducing the doping gas including a second amount of nitrogen into the molten mixture, a second ratio of the second amount of nitrogen to the doping gas being smaller than the first ratio; and upon occurrence of a shoulder of the monocrystalline silicon rod, adjusting the second amount of nitrogen to a third amount in such a manner that a third ratio of the third amount of nitrogen to the doping gas is greater than the second ratio, to form a monocrystalline silicon rod. A solar cell and a photovoltaic module including a gallium- and nitrogen-doped silicon wafer prepared therefrom are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Chinese Patent ApplicationNo. 202010624020.1, filed on Jun. 30, 2020, the content of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the technical field of a solar celland, in particular, to a method for preparing a monocrystalline silicon,a solar cell and a photovoltaic module including the preparedmonocrystalline silicon.

BACKGROUND

In existing solar cells, performances of the monocrystalline silicon andthe solar cells are usually improved by doping elements into themonocrystalline silicon. For P-type Czochralski monocrystalline silicon,it is usually doped with III group elements, such as B, Ga, and In. B isusually selected as the doping element in the monocrystalline silicon,and then in a Czochralski process, it is likely to form a boron-oxygen(B—O) complexes, which will cause light induced degradation of the solarcell and thus limit a conversion efficiency of the solar cell. For themonocrystalline silicon wafer, it is a tendency to develop larger size,thinner silicon wafer, and higher mechanical strength. High mechanicalstrength facilitates the silicon wafer processing. Therefore, it isnecessary to develop a silicon wafer that can reduce the light induceddegradation of the solar cell and have high mechanical strength, so asto facilitate production of large-sized and thin silicon wafers andimprove the conversion efficiency of subsequent cells.

SUMMARY

In view of the problems in the related art, the present disclosureprovides a method for preparing a monocrystalline silicon using aCzochralski process.

The method includes: obtaining a mixture by mixing a doping sourceincluding gallium and a polycrystalline silicon; feeding the mixtureinto a crucible of a single crystal furnace; introducing a doping gas atleast including a first amount of nitrogen into the mixture melt by thesingle crystal furnace, with a first ratio of the first amount ofnitrogen to the doping gas being greater than or equal to 0.8; providinga seed to the molten mixture; withdrawing the seed from the moltenmixture while introducing the doping gas including a second amount ofnitrogen into the molten mixture, with a second ratio of the secondamount of nitrogen to the doping gas being smaller than the first ratio;and forming, under an atmosphere of the doping gas, a monocrystallinesilicon rod by crystallizing the molten mixture around and below theseed as the seed is continually withdrawn from the molten mixture,during which upon occurrence of a shoulder of the monocrystallinesilicon rod, the second amount of nitrogen is adjusted to a third amountin such a manner that a third ratio of the third amount of nitrogen tothe doping gas is greater than the second ratio.

During melting of the mixture, by controlling the a first ratio of thefirst amount of nitrogen to the doping gas to be greater than or equalto 0.8, the nitrogen element is fully incorporated into the silicon, toform the monocrystalline silicon doped with nitrogen element.

It should be noted that, when withdrawing the seed from the moltenmixture, reaction of the silicon in the silicon melt will occur due toan excessive high concentration of the nitrogen in the single crystalfurnace, generating Si₃N₄ particles. The Si₃N₄ particles will causediscontinuities of the prepared monocrystalline silicon rod, resultingin defects in the monocrystalline silicon rod. In order to avoid that,the ratio of the nitrogen needs to be adjusted to a relatively lowvalue. Preferably, when withdrawing the seed from the molten mixture,the ratio of nitrogen in the doping gas (the second ratio) is smallerthan or equal to 0.1, to ensure smooth seeding and improve the qualityof the monocrystalline silicon rod.

In the step of forming the monocrystalline silicon rod by crystallizingthe molten mixture around and below the seed as the seed is continuallywithdrawn from the molten mixture, the ratio of nitrogen in the dopinggas is adjusted to be higher, to allow more nitrogen elements to befully incorporated into the monocrystalline silicon rod. Optionally, theratio of nitrogen in the doping gas is greater than or equal to 0.8.

Optionally, the formed monocrystalline silicon rod comprises dopantsincluding the gallium and the nitrogen, a doping concentration of thegallium being in a range of 0.001 Ω·cm to 100 Ω·cm and a dopingconcentration of the nitrogen being in a range of 0.01 ppma to 0.1 ppma.

Optionally, a doping concentration of the gallium is in a range of 0.001Ω·cm to 10 Ω·cm and a doping concentration of the nitrogen is in a rangeof 0.01 ppma to 0.1 ppma.

Optionally, in the method of the present disclosure, the doping sourceincluding gallium is composed of pure gallium materials. The puregallium materials can avoid the possibility of subsequent generation ofimpurities such as gallium nitride in a gallium-containing dopingsource, which will affect the quality of the prepared monocrystallinesilicon rod.

Optionally, the pure gallium materials are placed on a shoulder of thesingle crystal furnace to be melted and added. Since the pure galliummaterials have a small segregation coefficient, it is easy tovolatilize, and therefore addition of the pure gallium materials cancompensate for loss of the gallium, to improve a utilization rate of theraw materials and reduce production costs.

It should be noted that, the doping gas can be pure nitrogen, or a mixedgas of nitrogen and argon, and the nitrogen and the argon are introducedinto the single crystal furnace in at least one of the followingmanners:

(1) introducing the nitrogen and the argon from independent nitrogen andargon sources connected with independent mass flow meters respectivelyinto the single crystal furnace through a three-way pipeline, and usingmass flow meters to respectively control flow rates of the argon and thenitrogen;

(2) introducing the nitrogen and the argon from the independent nitrogenand argon sources into the single crystal furnace through an adjustablenitrogen inlet and an adjustable argon inlet provided on the singlecrystal furnace; or

(3) mixing the nitrogen and the argon from the independent nitrogen andargon sources in a predetermined ratio, and then introducing the mixednitrogen and argon gas into the single crystal furnace.

The present disclosure further provides a solar cell, and the solar cellincludes a gallium- and nitrogen-doped silicon wafer, and the gallium-and nitrogen-doped silicon wafer is prepared by the method above.

In addition, the present disclosure further provides a photovoltaicmodule, the photovoltaic module includes a solar cell string, the solarcell string includes a plurality of solar cells, and each of theplurality of solar cells includes the gallium- and nitrogen-dopedsilicon wafer prepared by the method above.

The present disclosure, through introducing the nitrogen during theprocess of forming the Czochralski monocrystalline silicon rod usinggallium-doped polycrystalline silicon materials, allows the nitrogenelement to enter the monocrystalline silicon rod to prepare Czochralskimonocrystalline silicon rod doped with both gallium and nitrogen, whichnot only solves a light attenuation problem of p-type monocrystals and aresistivity distribution problem of gallium monocrystals, but alsogreatly improves the mechanical strength of the monocrystalline siliconwafer. In addition, the doping of the nitrogen element can also improvedefect distribution in the monocrystals, increase the mechanicalstrength of the monocrystalline silicon, and improve the product qualityof the monocrystalline silicon.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is flowchart for preparing a monocrystalline silicon using aCzochralski (Cz) process according to an embodiment of the presentdisclosure; and

FIG. 2 is a flowchart for preparing an exemplary monocrystalline siliconusing a Czochralski process according to another embodiment of thepresent disclosure.

DESCRIPTION OF EMBODIMENTS

In order to make the purpose, technical solutions, and advantages of thepresent disclosure clearer, the present disclosure will be furtherdescribed below in detail with reference to the drawings andembodiments. It should be understood that the specific embodimentsdescribed here are only used to explain the present disclosure, and notto limit the present disclosure.

It should be understood that the term “and/or” used in the context ismerely an association relationship describing associated objects. Itmeans that there can be three kinds of relationships, for example, Aand/or B may mean: A alone exists, both A and B exist, and B aloneexists.

It should be understood that the terminology such as “on” and “under”described in the embodiments of the present disclosure are describedfrom the perspective of the accompanying drawings and should not beunderstood as a limitation to the embodiments of the present disclosure.In addition, it should also be understood that when it is mentioned thatone element is connected “on” or “under” another element, the oneelement can not only be directly connected “on” or “under” the anotherelement, but also can be indirectly connected “on” or “under” theanother element through an intermediate element. Unless otherwise notedin the context, the singular form expressions “a”, “an” “the” and “said”used in the embodiments and appended claims of the present disclosureare also intended to represent plural forms.

If there is no special statement, all technical features and preferredfeatures mentioned herein can be combined with each other to form a newtechnical solution. Unless otherwise defined or illustrated,professional and scientific terms used herein have the same meanings asthose familiar to those skilled in the art.

In the present disclosure, unless otherwise specified, a numerical range“a-b” represents an abbreviated representation of a set of real numbersbetween a and b, including a and b, and both a and b are real numbers.For example, a numerical range “800-900” means that all real numbersbetween 800 and 900 have been listed herein, and “800-900” is only anabbreviated representation of the set of these numerical values.

The “range” disclosed in the present disclosure is in a form of lowerand upper limits, which may include one or more lower limits and one ormore upper limits, respectively.

FIG. 1 is a flowchart for preparing a monocrystalline silicon using aCzochralski (Cz) process according to an embodiment of the presentdisclosure. In some embodiments, the monocrystalline silicon includedopants composed of gallium (Ga) and nitrogen (N) elements.

As shown in FIG. 1, the following steps are included:

step S01: mixing a gallium-containing doping source with apolycrystalline silicon raw material, putting an obtained mixture into acrucible and delivering the crucible with the obtained mixture into asingle crystal furnace together;

step S02: evacuating the single crystal furnace, and introducing a mixedgas of nitrogen and argon in different mixing ratios at differentpulling stages of Czochralski process;

and

step S03: pulling(or withdrawing) a crystal by the Czochralski processto obtain a gallium- and nitrogen-doped monocrystalline silicon rod.

Exemplary Czochralski process may include a plurality of processingstages, for example, charging, silicon melting, seeding, necking,shouldering, crystal body growth, and tailing. Specifically, a heatedcrucible holds a melted form of a charged material from which thecrystal is to be grown. A seed is placed at the end of a cable or rodthat will enable the seed to be lowered into the melt material and thenraised back out of the melt material. When the seed is lowered into themelt material, it causes a local decrease in melt temperature, whichresults in a portion of the melt material crystallizing around and belowthe seed. Thereafter, the seed is slowly withdrawn from the meltmaterial. As the seed is withdrawn or pulled from the melt material, theportion of the newly formed crystal that remains within the meltmaterial essentially acts as an extension of the seed and causes meltmaterial to crystallize around and below it. This process continues asthe crystal is withdrawn or pulled from the melt material, resulting incrystal body growth as the seed is continually raised. The singlecrystal silicon rod may be produced accordingly.

FIG. 2 is a flowchart for preparing an exemplary monocrystalline siliconusing a Czochralski process according to another embodiment of thepresent disclosure. In some embodiments, the monocrystalline siliconinclude dopants composed of gallium (Ga) and nitrogen (N) elements.

As shown in FIG. 2, the following steps are included:

step S10: obtaining a mixture by mixing a doping source includinggallium and a polycrystalline silicon. The doping source and thepolycrystalline silicon may be mixed in accordance with a predeterminedratio. The predetermined ratio relies on a desired resistivity of thepulled monocrystalline silicon with the Czochralski process.

step S20: feeding the mixture into a crucible of a single crystalfurnace (e.g., on the charging stage of the Cz process).

step S30: introducing a doping gas at least including a first amount ofnitrogen into the mixture melt by the single crystal furnace (e.g., onthe silicon melting stage of the Cz process), with a first ratio of thefirst amount of nitrogen to the doping gas being greater than or equalto 0.8.

step S40: providing a seed to the molten mixture. For example, the seedis placed at the end of a cable or rod in the single crystal furnace,which will enable the seed to be lowered into the melt material.

step S50: withdrawing the seed from the molten mixture (e.g., on theseeding stage of the Cz process) while introducing the doping gasincluding a second amount of nitrogen into the molten mixture, with asecond ratio of the second amount of nitrogen to the doping gas beingsmaller than the first ratio. In some embodiments, the second ratio maybe smaller than or equal to 0.1.

step S60: forming, under an atmosphere of the doping gas, amonocrystalline silicon rod by crystallizing the molten mixture aroundand below the seed as the seed is continually withdrawn from the moltenmixture (e.g., on the necking, shouldering, and/or tailing stages of theCz process). In some embodiments, on the shouldering stage, uponoccurrence of a shoulder of the monocrystalline silicon rod, the secondamount of nitrogen is adjusted to a third amount in such a manner that athird ratio of the third amount of nitrogen to the doping gas is greaterthan the second ratio. In some embodiments, the third ratio is greaterthan or equal to 0.8.

A gallium- and nitrogen-doped monocrystalline silicon rod, i.e.,gallium- and nitrogen-doped Czochralski monocrystalline silicon rod, canbe prepared by the above method(s) illustrated in FIG. 1 or FIG. 2. Agallium- and nitrogen-doped monocrystalline silicon wafer can beobtained by cutting the prepared gallium- and nitrogen-dopedmonocrystalline silicon rod. Each silicon rod can be cut to a pluralityof slices, i.e., silicon wafers. The doped gallium basically eliminateslight induced degradation of solar cells including the gallium- andnitrogen-doped monocrystalline silicon wafer, and the doped nitrogen canimprove mechanical properties of the crystalline silicon and enable itto have better mechanical strength, which is convenient for subsequentprocessing of the silicon rod, such as improving a yield of siliconwafers, making it easy to prepare silicon wafer products having largeand thin dimensions.

In the prepared gallium- and nitrogen-doped monocrystalline silicon rod,the doping concentration of gallium is in a range from 0.001 Ω·cm to 100Ω·cm, and the doping concentration of nitrogen is in a range from 0.01ppma to 0.1 ppma. Preferably, the doping concentration of gallium can bein a range from 0.001 Ω·cm to 10 Ω·cm, and the doping concentration ofnitrogen can be in a range from 0.01 ppma to 0.1 ppma.

Merely for illustration, the gallium- and nitrogen-doped monocrystallinesilicon rod can be prepared with reference to the following exemplaryembodiments. However, it can be understood that the followingembodiments are only exemplary and do not constitute limitations.

Embodiment 1

The present embodiment provides a method for preparing a gallium- andnitrogen-doped monocrystalline silicon, including following steps:

(1) On the charging stage, a gallium-containing doping source and apolycrystalline silicon material can be mixed and fed into a crucible ofa single crystal furnace. For example, the mixed materials of the puregallium and the polycrystalline silicon are placed in a quartz cruciblein the single crystal furnace. In some embodiments, thegallium-containing doping source includes but not limited to puregallium materials, gallium-containing compounds, gallium-containingalloys, or the like. In some embodiments, in case that other impuritiesin the doping source are introduced in the crystal withdrawing,preferably, the gallium-containing doping source is composed of puregallium materials.

In some examples, the gallium-containing doping source and thepolycrystalline silicon material can be added in various ways. Forexample, they can be added through a re-charging tube, and multiplere-charging (that is, the RCZ technology) is currently one of theproduction technologies commonly used by manufacturers of themonocrystalline silicon. Based on the traditional process of pulling onemonocrystalline silicon rod once per run, the multiple re-chargingprocess is a process in which after the first monocrystalline siliconrod (with a certain amount of molten silicon remaining in the crucible)is pulled out, the polycrystalline silicon is recharged into thecrucible through the re-charging tube, and then the second, third oreven more monocrystalline silicon rod is pulled similarly as the firstone. This process increases the capacity in a single run and reducestime for blow-out and blow-in the furnace, thereby increasing theproportion of pulling time in the total operating time, and the costs ofquartz crucible and crystal pulling are reduced. For example, thegallium-containing doping source and the polycrystalline siliconmaterial may be added through an external feeder. The external feeder isa material feeding device used in a Continuous Czocharlski method (i.e.,CCZ method). In the continuous Czochralski method, the withdrawing ofthe crystal rod and the material feeding and melting are performedsimultaneously. The single crystal furnace of the CCZ method is equippedwith a funnel for storing silicon raw material particles and the funnelis connected to a vibrating feeder. While withdrawing the crystal rod,the raw material is continuously added through the external feeder tothe crucible to be melted, so as to realize the continuous withdrawingof the monocrystalline silicon rod. The use of the CCZ method and theexternal feeder greatly improves a production efficiency.

(2) On the silicon melting stage, the silicon is melted by the singlecrystal furnace, during which a doping gas containing at least nitrogenis introduced, and a first ratio of the nitrogen to the doping gas isgreater than or equal to 0.8. For example, the first ratio is 0.8 to1.0. In some examples, the doping gas can also function as a protectivegas. In some embodiments, the doping gas further includes argon. Thedoping gas can be designated as a mixed gas including the nitrogen gasand the argon gas.

In some examples, the doping gas is introduced into the single crystalfurnace by at least one of the following manners:

1) The nitrogen and the argon from independent nitrogen and argonsources connected with independent mass flow meters respectively areintroduced into the single crystal furnace through a three-way pipeline,and the mass flow meters are used to respectively control flow rates ofthe argon and the nitrogen. The three-way pipeline may be a three-wayvalve device, and the three-way valve device is at least partiallyarranged outside the furnace.

2) The nitrogen and the argon from the independent nitrogen and argonsources are introduced into the single crystal furnace through anadjustable nitrogen inlet and an adjustable argon inlet provided on thesingle crystal furnace;

3) The nitrogen and the argon from the independent nitrogen and argonsources are mixed in a predetermined ratio, and then the mixed nitrogenand argon gas is introduced into the single crystal furnace. Forexample, the predetermined ratio is the first ratio described above. Insome examples, in the stage of feeding material to the crucible of thesingle crystal furnace, the nitrogen and the argon from the independentnitrogen and argon sources are mixed in a ratio of 0.8, 085, 0.9, or0.95 (nitrogen/(argon+nitrogen)), and then the mixed nitrogen and argongas are introduced into the single crystal furnace. In a stage ofwithdrawing the seed from the molten mixture, the nitrogen and the argonfrom the independent nitrogen and argon sources are mixed in a ratio of0.05 or 0.1 (nitrogen/(argon+nitrogen)), and then the mixed nitrogen andargon gas is introduced into the single crystal furnace. The mixed gascontaining nitrogen and argon that have been mixed in the predeterminedratio is introduced into the single crystal furnace, and a stable gasatmosphere can be therefore formed, which is conducive to obtaining amonocrystalline silicon rod of uniform quality.

In an example, the single crystal furnace is evacuated, and then thenitrogen and the argon from independently adjustable nitrogen and argonsources are introduced into the single crystal furnace through thethree-way pipeline, and the ratio of the nitrogen to the total gas isadjusted to 0.8. During melting the polysilicon materials, heating isperformed with a heater, a rotation speed of the quartz crucible isadjusted, and a pressure in a cavity of the single crystal furnace ismaintained at 13 Torr by controlling a throttle opening of a vacuumpump.

When the silicon is being melt, attention should be paid to check ifthere are Si₃N₄ particles formed. If so, slag handling is carried out intime.

(3) On the stage of the seeding, a seed is provided to the moltenmixture, and then withdrawn from the molten mixture while introducingthe doping gas. In this process, the ratio of nitrogen to the doping gasis adjusted to a second ratio, and the second ratio is smaller than thefirst ratio. Optionally, the second ratio is smaller than or equal to0.1. It should be noted that during the seeding process, it is necessaryto maintain a relatively small amount of nitrogen, to avoid theformation of the Si₃N₄ particles. If the Si₃N₄ particles are formed, theslag handling is carried out in time.

In some examples, the ratio of the nitrogen is gradually reduced throughthe mass flow meter connected with the nitrogen source, and theprotective gas in the single crystal furnace is replaced with argon;then a nitrogen flow is increased gradually, to adjust the ratio of thenitrogen in the protective gas (the nitrogen and the argon) to 0.1; themolten silicon is thermally insulated, to ensure that the temperatureand flowing of the molten silicon are stable; a seed crystal is fixed ona seed shaft to rotate; the seed crystal is slowly lowered, and pausedat a certain distance (about 10 mm) from a liquid surface of the moltensilicon, and when the temperature of the seed crystal is close to thetemperature of the silicon melt, the seed crystal is gently immersed inthe silicon melt; and then the seed crystal is lifted and withdrawn fromthe melt to form the monocrystalline silicon.

(4) On the stage of the necking, the seed crystal is up-lifted quickly,to make a diameter of newly crystallized monocrystalline silicon reach 3mm, a length be about 6-10 times of the diameter of the crystal at thistime; and the rotation speeds of the seed crystal and the quartzcrucible are adjusted, to reduce a local diameter of the monocrystallinesilicon.

(5) On the stage of the shouldering, the speed for withdrawing the seed(also referred to as pulling speed) is reduced to control the diameterof the crystal to a desired size. On this stage, a doping sourceincluding gallium is added at the same time. Preferably, the dopingsource including gallium is composed of pure gallium materials. The puregallium materials can avoid the possibility of subsequent generation ofimpurities such as gallium nitride in a gallium-containing dopingsource, which will affect the quality of the prepared monocrystallinesilicon rod. In some examples, since the volatility coefficient ofgallium is relatively large, it is easy to result in volatilization ofthe gallium material, and in order to compensate for the loss of thegallium material, the elemental gallium is placed on a shoulder of themonocrystalline silicon rod to be re-melted and re-added.

(6) Upon occurrence of a shoulder of the monocrystalline silicon rod(e.g., during the body growth and the tailing stages), the nitrogen inthe doping gas is adjusted to a third ratio, the third ratio beinglarger than the second ratio in the seeding stage. For example, theratio of the nitrogen in the protective gas (nitrogen and argon) isgradually adjusted to be greater than or equal to 0.8 by the mass flowmeters; then the rotation speed of the crystal is adjusted to 3-12 rpm,and a reverse rotation speed of the quartz crucible is adjusted to 2-10rpm, to allow the crystal to grow with a constant diameter.

(7) Under the atmosphere of the doping gas with the third ratio, thediameter of the crystal gradually decreases as the crystal is liftedfrom the melt, the temperature is lowered, cooling is performed, and themonocrystalline silicon rod is taken out to obtain a gallium- andnitrogen-doped monocrystalline silicon rod.

After the tailing stage, the crystal is cooled, the gallium- andnitrogen-doped monocrystalline silicon can be obtained, in which adoping concentration of gallium is 0.001 Ω·cm to 100 Ω·cm, and a dopingconcentration of nitrogen is 0.01 ppma to 0.1 ppma. Preferably, thedoping concentration of gallium is 0.001 Ω·cm to 10 Ω·cm, and the dopingconcentration of nitrogen is 0.01 ppma to 0.1 ppma.

Embodiment 2

The present embodiment provides the preparation of a gallium- andnitrogen-doped Czochralski monocrystalline silicon rod by charging thematerials using the re-charging tube.

(1) On the charging stage, elemental gallium as a dopant and apolycrystalline silicon material are mixed, the mixed material is placedin the quartz crucible and the quartz crucible together with the mixedmaterial is delivered to a re-feeding single crystal furnace. The mixedmaterial of the polycrystalline silicon and the elemental gallium needsto be placed in the quartz crucible.

(2) On the silicon melting stage, the single crystal furnace isevacuated, then nitrogen and argon from independently adjustablenitrogen and argon sources are introduced into the single crystalfurnace through a three-way valve, and the ratio of the nitrogen to themixed gas (nitrogen and argon) is adjusted to 0.8. When beginningmelting of the silicon, heating is performed with a heater, a rotationspeed of the quartz crucible is adjusted. A pressure in a cavity of thesingle crystal furnace is maintained at 0.7 kPa by controlling athrottle opening of a vacuum pump.

When melting silicon, attention should be paid to check if there areSi₃N₄ particles formed. If so, slag handling is carried out in time.

(3) On the stage of the seeding, the ratio of the nitrogen is graduallyreduced through mass flow meters connected to the independentlyadjustable nitrogen and argon sources, the protective gas in the singlecrystal furnace is replaced with argon, and a nitrogen flow is graduallyreduced, to adjust the ratio of the nitrogen in the protective gas(nitrogen and argon) to 0.1; the molten silicon is thermally insulated,to ensure that a temperature and flowing of the molten silicon arestable; a monocrystalline seed crystal is fixed on a seed crystal shaftto rotate with the seed crystal shaft; the seed crystal is slowlylowered, and paused at about 10 mm from a liquid surface of the moltensilicon, and when the temperature of the seed crystal is close to thetemperature of the silicon melt, the seed crystal is gently immersed inthe silicon melt; and then the seed crystal is lifted and withdrawn fromthe silicon melt to form the monocrystalline silicon.

(4) On the stage of the necking, the seed crystal is lifted quickly, tomake a diameter of newly crystallized monocrystalline silicon reach 3mm, a length be about 6-10 times of the diameter of the crystal at thistime; and the rotation speeds of the seed crystal and the quartzcrucible are adjusted, to reduce a local diameter of the monocrystallinesilicon.

(5) On the stage of the shouldering, the speed for withdrawing the seedis reduced, to control the diameter of the crystal to a desired size.

(6) Upon occurrence of a shoulder of the monocrystalline silicon rod,the ratio of the nitrogen in the protective gas (nitrogen and argon) isgradually adjusted to be 0.9 by the mass flow meters; and then therotation speed of the crystal is adjusted to 3-12 rpm, and a reverserotation speed of the quartz crucible is adjusted to 2-10 rpm, to allowthe crystal to grow with a constant diameter.

(7) Under the atmosphere of the doping gas with the ratio of 0.9, thediameter of the crystal gradually decreases as the crystal is liftedfrom the melt; the temperature is lowered, cooling is performed, and themonocrystalline silicon rod is taken out, to obtain a gallium- andnitrogen-doped monocrystalline silicon rod in which a dopingconcentration of gallium is 10 Ω·cm and a doping concentration ofnitrogen is 0.1 ppma.

(8) Re-charging and crystal pulling: after taking out a monocrystallinesilicon rod segment obtained in step (7), the re-charging tubecontaining the gallium and the polycrystalline silicon raw material isplaced in an auxiliary chamber of the single crystal furnace, and a gatevalve is then closed. In a secondary feeding process, a small-particlepolycrystalline silicon material is selected for re-charging. At thesame time, the power of the heater in the single crystal furnace isreduced to 40-45 kW, the surface temperature of the remaining melt inthe quartz crucible was reduced to about 1400° C. and crystallizationoccurred. The rotation speed of the crucible is reduced, the gate valveis opened, the re-charging tube enters a main chamber of the singlecrystal furnace, the re-charging tube is lowered to be above the quartzcrucible, and a feeding port of the re-charging tube is opened, to allowthe polycrystalline silicon raw material to be poured into the quartzcrucible. The above steps (1) to (7) are repeated until multiplemonocrystalline silicon rods are pulled.

The multiple re-charging method of the Czochralski monocrystallinesilicon raw material of the present embodiment can increase theproportion of pulling time in the total running time of the furnace,greatly improve the production efficiency, reduce the production cost ofmonocrystalline silicon, and provide strong technical support for themanufacture of monocrystalline silicon and solar cells.

Comparative Example

The steps of this comparative example are the same as those inEmbodiment 2, except that: in the seeding operation of step (3), theratio of the nitrogen in the protective gas was not reduced. In theseeding stage, the ratio of the nitrogen in the protective gas is still0.8. Under this condition, the inventor found that a large number ofparticles were generated on the molten silicon surface, which weredetected as Si₃N₄ particles. It is found that Si₃N₄ particles are causedby the reaction between silicon and nitrogen due to the highconcentration of nitrogen in the process of seeding, resulting in theformation of fine Si₃N₄ particles. Once the Si₃N₄ particles floating onthe surface of melt come into contact with the growing monocrystallinesilicon, it will lead to the broken ridge of single crystal and thuslose its structure. After research and adjustment, the ratio of nitrogenin the process of seeding in the protective gas is adjusted to a lowervalue, which can avoid the single crystal broken ridge caused by theformation of Si₃N₄ particles.

Because the monocrystalline silicon rod prepared by the method of thepresent disclosure is doped with nitrogen in a certain ratio, thesilicon rod has relatively high mechanical strength, and since thesilicon rod is doped with the gallium, the light induced degradation ofa solar cell can be reduced, which is beneficial to improve a conversionefficiency of the solar cell and achieve large size and thinning of thesilicon wafer.

In some embodiments, a solar cell can be prepared based on themonocrystalline silicon. The solar cell includes a gallium- andnitrogen-doped silicon wafer, and the gallium- and nitrogen-dopedsilicon wafer can be obtained by cutting the gallium- and nitrogen-dopedsilicon rod prepared by the method described above. For example, thesolar cell is formed by performing texturing, diffusion, laser SE,etching, thermal oxygenation, passivation treatment, screen printing,and sintering for the gallium- and nitrogen-doped silicon wafer. Thesolar cell includes the gallium- and nitrogen-doped silicon wafer, andpassivation layers and electrodes on front and back sides of thegallium- and nitrogen-doped silicon wafer.

In some embodiments, a photovoltaic module can be formed. Thephotovoltaic module includes at least one solar cell string. The solarcell string includes a plurality of solar cells, which each includes thegallium- and nitrogen-doped silicon wafer obtained by cutting thegallium- and nitrogen-doped silicon rod prepared by the above method.For example, at least one solar cell string each composed of theprepared solar cells is obtained, a welding tape is used to weld thecells in series, and through a lamination process, a back plate,ethylene-vinyl acetate copolymer (EVA) and the solar cells are laminatedin a certain order, subsequently, the laminated structure isencapsulated, and a border is instated to form the photovoltaic module.The solar cell can convert received light energy into electrical energy.The photovoltaic module transfers the electrical energy to a load (e.g.,an inverter).

The above descriptions are only preferred examples of the presentdisclosure and are not intended to limit the present disclosure. Forthose skilled in the art, the present disclosure can have variousmodifications and changes. Any modification, equivalent replacement,improvement, etc. made within the principle of the present disclosureshall be included in the protection scope of the present disclosure.

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What is claimed is:
 1. A method for preparing a monocrystalline siliconusing a Czochralski process, comprising: obtaining a mixture by mixing adoping source including gallium and a polycrystalline silicon; feedingthe mixture into a crucible of a single crystal furnace; introducing adoping gas at least including a first amount of nitrogen into themixture melt by the single crystal furnace, wherein a first ratio of thefirst amount of nitrogen to the doping gas is greater than or equal to0.8; providing a seed to the molten mixture; withdrawing the seed fromthe molten mixture while introducing the doping gas including a secondamount of nitrogen into the molten mixture, wherein a second ratio ofthe second amount of nitrogen to the doping gas is smaller than thefirst ratio; and forming, under an atmosphere of the doping gas, amonocrystalline silicon rod by crystallizing the molten mixture aroundand below the seed as the seed is continually withdrawn from the moltenmixture, wherein upon occurrence of a shoulder of the monocrystallinesilicon rod, the second amount of nitrogen is adjusted to a third amountin such a manner that a third ratio of the third amount of nitrogen tothe doping gas is greater than the second ratio.
 2. The method accordingto claim 1, wherein the formed monocrystalline silicon rod comprisesdopants including the gallium and the nitrogen, a doping concentrationof the gallium being in a range of 0.001 Ω·cm to 100 Ω·cm and a dopingconcentration of the nitrogen being in a range of 0.01 ppma to 0.1 ppma.3. The method according to claim 1, wherein the formed monocrystallinesilicon rod comprises dopants including the gallium and the nitrogen, adoping concentration of the gallium being in a range of 0.001 Ω·cm to 10Ω·cm and a doping concentration of the nitrogen being in a range of 0.01ppma to 0.1 ppma.
 4. The method according to claim 1, wherein the secondratio is smaller than or equal to 0.1.
 5. The method according to claim1, wherein the third ratio is greater than or equal to 0.8.
 6. Themethod according to claim 1, wherein the doping source including galliumis composed of pure gallium materials.
 7. The method according to claim1, wherein the doping gas further comprises argon, wherein the nitrogenand the argon are introduced into the single crystal furnace through atleast one of: (1) introducing the nitrogen and the argon fromindependent nitrogen and argon sources connected with independent massflow meters respectively into the single crystal furnace through athree-way pipeline, and using the mass flow meters to respectivelycontrol flow rates of the argon and the nitrogen; (2) introducing thenitrogen and the argon from the independent nitrogen and argon sourcesinto the single crystal furnace respectively through an adjustablenitrogen inlet and an adjustable argon inlet provided on the singlecrystal furnace; or (3) mixing the nitrogen and the argon from theindependent nitrogen and argon sources in a predetermined ratio, andintroducing the mixed nitrogen and argon gas into the single crystalfurnace.
 8. A solar cell comprising a gallium- and nitrogen-dopedsilicon wafer, wherein the gallium- and nitrogen-doped silicon wafer hasa doping concentration of gallium in a range of 0.001 Ω·cm to 100 Ω·cmand a doping concentration of nitrogen in a range of 0.01 ppma to 0.1ppma.
 9. The solar cell according to claim 8, wherein the gallium- andnitrogen-doped silicon wafer has a doping concentration of the galliumin a range of 0.001 Ω·cm to 10 Ω·cm and a doping concentration ofnitrogen in a range of 0.01 ppma to 0.1 ppma.
 10. The solar cellaccording to claim 8, wherein the gallium- and nitrogen-doped siliconwafer is prepared using a Czochralski process comprising: obtaining amixture by mixing a doping source including gallium and apolycrystalline silicon; feeding the mixture into a crucible of a singlecrystal furnace; introducing a doping gas at least including a firstamount of nitrogen into the mixture melt by the single crystal furnace,wherein a first ratio of the first amount of nitrogen to the doping gasis greater than or equal to 0.8; providing a seed to the molten mixture;withdrawing the seed from the molten mixture while introducing thedoping gas including a second amount of nitrogen into the moltenmixture, wherein a second ratio of the second amount of nitrogen to thedoping gas is smaller than the first ratio; forming, under an atmosphereof the doping gas, a monocrystalline silicon rod by crystallizing themolten mixture around and below the seed as the seed is continuallywithdrawn from the molten mixture, wherein upon occurrence of a shoulderof the monocrystalline silicon rod, the second amount of nitrogen isadjusted to a third amount in such a manner that a third ratio of thethird amount of nitrogen to the doping gas is greater than the secondratio; and cutting the formed monocrystalline silicon rod to form thegallium- and nitrogen-doped silicon wafer.
 11. The solar cell accordingto claim 10, wherein the second ratio is smaller than or equal to 0.1.12. The solar cell according to claim 10, wherein the third ratio isgreater than or equal to 0.8.
 13. The solar cell according to claim 10,wherein the doping source including gallium is composed of pure galliummaterials.
 14. The solar cell according to claim 10, wherein the dopinggas further comprises argon, wherein the nitrogen and the argon areintroduced into the single crystal furnace through at least one of: (1)introducing the nitrogen and the argon from independent nitrogen andargon sources connected with independent mass flow meters respectivelyinto the single crystal furnace through a three-way pipeline, and usingmass flow meters to respectively control flow rates of the argon and thenitrogen; (2) introducing the nitrogen and the argon from theindependent nitrogen and argon sources into the single crystal furnacerespectively through an adjustable nitrogen inlet and an adjustableargon inlet provided on the single crystal furnace; or (3) mixing thenitrogen and the argon from the independent nitrogen and argon sourcesin a predetermined ratio, and then introducing the mixed nitrogen andargon gas into the single crystal furnace.
 15. A photovoltaic modulecomprising a solar cell string, wherein the solar cell string comprisesa plurality of solar cells, and each of the plurality of solar cellscomprises a gallium- and nitrogen-doped silicon wafer, wherein thegallium- and nitrogen-doped silicon wafer has a doping concentration ofgallium in a range of 0.001 Ω·cm to 100 Ω·cm and a doping concentrationof nitrogen in a range of 0.01 ppma to 0.1 ppma.
 16. The photovoltaicmodule according to claim 15, wherein the gallium- and nitrogen-dopedsilicon wafer has a doping concentration of the gallium in a range of0.001 Ω·cm to 10 Ω·cm and a doping concentration of nitrogen in a rangeof 0.01 ppma to 0.1 ppma.
 17. The photovoltaic module according to claim15, wherein the gallium- and nitrogen-doped silicon wafer is preparedusing a Czochralski process comprising: obtaining a mixture by mixing adoping source including gallium and a polycrystalline silicon; feedingthe mixture into a crucible of a single crystal furnace; introducing adoping gas at least including a first amount of nitrogen into themixture melt by the single crystal furnace, wherein a first ratio of thefirst amount of nitrogen to the doping gas is greater than or equal to0.8; providing a seed to the molten mixture; withdrawing the seed fromthe molten mixture while introducing the doping gas including a secondamount of nitrogen into the molten mixture, wherein a second ratio ofthe second amount of nitrogen to the doping gas is smaller than thefirst ratio; forming, under an atmosphere of the doping gas, amonocrystalline silicon rod by crystallizing the molten mixture aroundand below the seed as the seed is continually withdrawn from the moltenmixture, wherein upon occurrence of a shoulder of the monocrystallinesilicon rod, the second amount of nitrogen is adjusted to a third amountin such a manner that a third ratio of the third amount of nitrogen tothe doping gas is greater than the second ratio; and cutting the formedmonocrystalline silicon rod to form the gallium- and nitrogen-dopedsilicon wafer.
 18. The photovoltaic module according to claim 17,wherein the second ratio is smaller than or equal to 0.1.
 19. Thephotovoltaic module according to claim 17, wherein the third ratio isgreater than or equal to 0.8.
 20. The photovoltaic module according toclaim 17, wherein the doping source including gallium is composed ofpure gallium materials.